A variety of optical communications modules exist for transmitting and/or receiving optical data signals over optical waveguides (e.g., optical fibers). Optical communications modules include optical receiver, optical transmitter and optical transceiver modules. Optical receiver modules have one or more receive channels for receiving one or more optical data signals over one or more respective optical waveguides. Optical transmitter modules have one or more transmit channels for transmitting one or more optical data signals over one or more respective optical waveguides. Optical transceiver modules have one or more transmit channels and one or more receive channels for transmitting and receiving respective optical transmit and receive data signals over respective transmit and receive optical waveguides. For each of these different types of optical communications modules, a variety of designs and configurations exist.
In optical receiver and transceiver modules, an optical data signal passing out of an end of an optical fiber is coupled by an optics system onto an optical detector, such as a P-intrinsic-N (PIN) diode or other type of photodiode. The photodiode converts the optical data signal into an electrical current signal, which is then converted into an electrical voltage signal, amplified and processed to recover the data. The current-to-voltage conversion and amplification processes are typically performed by a transimpedance amplifier (TIA) circuit.
In many cases, it is desirable or necessary to provide an indicator signal that is indicative of the optical power level of the incident light striking the photodiode. The indicator signal is typically referred to as a receiver signal strength indicator (RSSI) signal, and the signal may be either an analog or digital signal and may or may not be amplified. Known RSSI circuits exist for determining the optical power level of the incident light based on a measurement of the electrical current produced by the photodiode.
A typical RSSI circuit includes a filter circuit for filtering out high frequency noise applied to the photodiode by the supply voltage. The filter circuit typically includes a resistor and a capacitor connected in series. By sensing the voltage across the resistor, the input current signal output by the photodiode to the RSSI circuit is sensed. The input current signal is proportional to the input optical power, i.e., the optical power level of the light striking the photodiode. Hence, the RSSI circuit detects the input optical power.
FIG. 1 illustrates a block diagram of a typical RSSI circuit 1 for generating an electrical current signal proportional to the photocurrent produced by a photodiode 2 when light strikes the photodiode 2. The cathode of the photodiode 2 is connected to a supply voltage filter circuit comprising a first resistor, R1, 3 and a capacitor, CFLT, 4. The anode of the photodiode 2 is connected to an input of a TIA 6. The supply voltage filter circuit acts as a low-pass filter that removes high frequency noise from the supply voltage, VCC. The input current, IPIN, produced by the photodiode 2 flows through resistor R1 3, which generates a time-varying voltage signal that is dependent on IPIN. Because the RC time constant associated with R1 and CFLT is much larger than the data rate of the RSSI circuit 1, the voltage, V1, across R1 varies very little with time and is therefore useful in tracking the average input current, which is calculated as V1/R1.
An operational amplifier (op-amp) 5, a second resistor, R2, 7 and a p-type metal oxide semiconductor transistor (PMOS) 8 are used to generate an output current, IOUT, proportional to the average input current IPIN*(R1/R2), where the symbol “*” represents a multiplication operation. The RSSI circuit 1 will force the same voltage V1 that is across R1 to be across R2, creating a current in R2 equal to V1/R2 that flows in and out of the PMOS 8 and into an appropriate load 9 having a load impedance, ZLOAD. The output current, IOUT, which equals IPIN*(R1/R2), is normally considered the RSSI signal and this signal is typically used by other circuitry (not shown) to monitor the optical power level of the photodiode 2. In some cases, the RSSI signal is amplified and/or digitized.
With RSSI circuits having the configuration shown in FIG. 1, the voltage bias from the supply voltage VCC that is applied to the photodiode 2 must be kept above a minimum value in order for the photodiode 2 to operate properly. This minimum voltage value for the photodiode 2 limits the allowed voltage drop V1 across R1, which constrains the value of R1 to a small value. If the value of R1 is too large, the voltage drop V1 will not be sufficiently below VCC to ensure that the voltage bias applied to the photodiode 2 is large enough for it to operate properly. Therefore, when using the RSSI circuit configuration shown in FIG. 1, the value of R1 must be chosen so that it is small enough to support maximum current flow and to maintain a voltage level across R1 that is sufficiently below VCC. On the other hand, a large value for R1 is desired in order to reduce the low-pass bandwidth of the filter circuit and to provide a signal that is sufficiently large to allow accurate sensing of the input current signal.
Accordingly, a need exists for an RSSI circuit that ensures that the impedance of the supply voltage filter circuit is small enough that the photodiode has adequate voltage to operate properly and large enough to ensure accurate sensing of the input current signal and effective supply voltage filtering.